Fact Check: Does COVID-19 Lose 90 Percent Infectiousness in 20 Minutes?

A new study suggests that COVID-19 particles lose virtually all of their infectiousness in 20 minutes in the air - but how true is the claim? Image Credit: Alexander Limbach/Shutterstock/IFLScience

It’s not often that good news comes out about the COVID-19 pandemic, but you may have noticed one piece making the headlines recently: apparently, a study has shown that the virus loses 90 percent of its infectious ability after 20 minutes in air. So, what's the science behind this, and how reliable is it?

“When you move further away [from somebody], not only is the aerosol diluted down, there’s also less infectious virus because the virus has lost infectivity [as a result of time],” study lead author Jonathan Reid, director of the University of Bristol’s Aerosol Research Centre, told The Guardian.

“It means that if I’m meeting friends for lunch in a pub today, the primary [risk] is likely to be me transmitting it to my friends, or my friends transmitting it to me, rather than it being transmitted from someone on the other side of the room.”

There are good reasons to be optimistic about these results. Our ideas until now about how the virus spreads in air were based on experiments with Goldberg drums – a kind of scientific tombola that spins continuously in order to keep the particles inside aerosolized. From those studies, researchers concluded that viral particles remained present in the air for at least three hours – results which informed much of the advice, good and bad, going forward.

However, the new paper used a completely different technique – one that Distinguished Professor at the University of Colorado Boulder Jose-Luis Jimenez, who was not involved in the research, described in tweets as “very elegant” and “outstanding.” The team developed specialized apparatus that allowed them to levitate particles between two electric rings for up to twenty minutes while tightly controlling the humidity, temperature, and UV light.

“This is the first time anyone has been able to actually simulate what happens to the aerosol during the exhalation process,” Reid told The Guardian. The sharp loss of infectivity, most of which occurs within the first five minutes, the team attributed to exposure to dry air and a rise in the pH of the virus: in a Goldberg drum, the authors explain, the “confined volume results in the elevation of the CO2 gas concentration […] [and] CO2 in the gas phase reduces the degradation of the virus […] by limiting the rise in droplet pH.”

In other words, those experiments that found SARS-CoV-2 particles could survive in the air for hours were supposedly completely sabotaging themselves. Instead of measuring how long the viral particles would stay infectious in the "real world", they were measuring how long they could keep the particles infectious using fancy lab equipment.

On the face of it, this is huge news. SARS-CoV-2, the virus that causes COVID-19 and its many variants of concern, is mainly spread via aerosol or droplet transmission – if hanging around in the air can effectively neuter it, maybe everything will be alright after all?

“People have been focused on poorly ventilated spaces and thinking about airborne transmission over meters or across a room,” said Reid. “I’m not saying that doesn’t happen, but I think still the greatest risk of exposure is when you’re close to someone.”


Among the scientific community, this paper is stirring quite a bit of controversy.

“I think the conclusions are overstretched,” Jimenez wrote. “One issue is that they are not real respiratory aerosols but surrogates, and we know that the chemical composition of the droplets is very important."

"Also this study suggests that infection should go down A LOT in dry periods. But [for example] we found in a study in Argentina … that DRY periods were the ones that had the most infection.”

“This paper is not published.. not reviewed.. and has serious problems that will hopefully be fixed during the review process,” commented Kimberley Prather, Distinguished Professor at UC San Diego and Director of the Center for Aerosol Impacts on Chemistry of the Environment. “The lead authors know this.”

Peer review is essentially the process by which scientists keep each other in check. If you’re reading a paper that hasn’t been peer-reviewed, you’re basically just taking the authors’ results on trust.

The new paper, as Prather pointed out, has not been peer reviewed – meaning nobody sufficiently trained to critique the results has had the chance to officially point out any mistakes, omissions, or personal biases the researchers might have brought to the experiment.

After two years of seemingly endless doom and gloom, it’s natural that we might jump at the chance for some optimism – even if it’s a bit of a long shot, scientifically speaking. In fact, on more than one occasion recently the search for good news has taken us away from peer-reviewed research altogether. Now, that’s not necessarily always a bad thing – it may well be the case that a paper is perfectly cromulent, but simply hasn’t been out long enough to be peer-reviewed – but it’s definitely an important thing.

“[The] experiments are great, but not fully realistic,” wrote Jimenez. “Not consistent with real-world [epidemiological] data. There is a lot of shared-air superspreading.”

“I would say the experiments could be useful if done to properly simulate the production process […] using more realistic fluid composition,” replied Prather, adding that the simulations in the paper were “not even close”.

Referencing the results section of the preprint, she pointed out that the authors “acknowledge they are not using realistic composition,” asking “what do their evaporation dynamics, structure, and infectious results even mean relative to the real world?”

Linsey Marr, Charles P. Lunsford Professor of Civil and Environmental Engineering at Virginia Tech and an expert in airborne transmission of viruses, had the same caution over the preprint’s results.

“Initial reactions (haven't read in detail yet): great methods, nice insight into dynamics at shorter time scales than before,” she tweeted. However, she added, “we have looked at several [different] viruses in droplets and aerosols, and decay is EXTREMELY sensitive to media composition.”

So what does all this mean with regards to infectiousness? Basically, just that it’s too early to take much from the study at all – it hasn’t been peer-reviewed, and (unless you’re a professor of epidemiology or virology reading this right now, in which case we’re honored) you and I simply aren’t qualified to draw any firm conclusions from it.

One thing is for certain, though: as Carl Sagan famously put it, “extraordinary claims require extraordinary evidence” – or in academic terms, “the reviewers have their work cut out for them,” tweeted Prather.

The authors of this preprint may have hit upon some exciting new results, and they’ve definitely impressed with their methodology – but while their conclusions seem so at odds with what experts currently believe, we should definitely be wary of any tabloid headlines using the paper to urge us back to the office prematurely.


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